In biennials and winter annuals, flowering is typically blocked in the first growing season. Exposure to the prolonged cold of winter, through a process called vernalization, is required to alleviate this block and permit flowering in the second growing season.
Plants have evolved the ability to alter their developmental program in response to environmental stimuli. A major switch in the developmental program is the transition to flowering. In many species the timing of this transition is determined by sensing seasonal changes. Photoperiod and temperature are two main environmental cues that plants monitor to determine the correct time to flower.
Vernalization is a term describing the promotion of flowering after exposure to cold. Specifically, vernalization results in “the acquisition or acceleration of the ability to flower by a chilling treatment” [31]. In other words, after vernalization plants do not necessarily initiate flowering but acquire the competence to do so. In many plant species, vernalization requires long-term exposure to the low temperatures of a typical winter. This is a useful adaptation because many vernalization-requiring species have a winter-annual or biennial habit; the plants begin growing in one season but flower in the spring of the second growing season. The term vernalization is derived from the Latin word vernus, meaning “of the spring”. In vernalization-requiring species it is critical that the plants are not “tricked” into flowering in the late autumn season by transient exposure to cold followed by warm conditions; thus the requirement for prolonged cold. Flowering of many vernalization-requiring species is also promoted by long photoperiods and this photoperiod requirement provides another level of assurance that flowering does not occur in late autumn when the days are short.
The physiology of vernalization has been extensively studied since the defining work of Gustav Gassner in early 20th century [discussed in 32]. Grafting and localized cooling studies show that the apical meristem is the site of cold perception during vernalization and that vernalization causes the meristem to become competent to flower [32, 33, 34]. Once meristems have been exposed to prolonged cold, they “remember” that they have been vernalized. This memory is mitotically stable. One of the classic experiments that demonstrated this memory was to vernalize biennial Hyoscyamus niger and subsequently grow the vernalized plants in non-inductive photoperiods [discussed in 32]. The vernalized H. niger plants were able to remember the prior vernalization for long periods of time and were subsequently able to flower when exposed to inductive photoperiods. Another classic study that demonstrated both the site of vernalization and the memory effect was the in vitro regeneration of plants from various tissues of vernalized Lunaria biennis [33, 34]. Only tissues that contained dividing cells (including root meristems) regenerated into vernalized plants. Thus dividing cells (or perhaps cells in which DNA replication is occurring) are a prerequisite for vernalization, and the vernalized state is maintained through tissue culture. This type of experiment has also been done in Arabidopsis [35]. The mitotically stable cellular memory illustrates the epigenetic nature of vernalization. Of course it is vital that this memory is lost in the next generation so that the vernalization requirement is re-established.
These classical studies of vernalization raise some interesting questions. How can plants measure long-term cold exposure? For example, why does a week of cold not result in vernalization when 4 weeks does? What is the basis of this mitotically stable cellular memory of vernalization.
Recent genetic and physiological studies of the vernalization pathway in Arabidopsis and the identification of components involved in this pathway provide a framework for addressing these intriguing questions. As well, these discoveries provide useful molecules and methods for inducibly conferring epigenetic change beyond the vernalization context. Such, techniques for providing permanent change in gene expression of preselected genes would be widely welcomed in a variety of fields, including agriculture and pharmaceutical sciences.